1. Technical Field
The present invention relates to an ink composition for an ink jet.
2. Related Art
An ink jet recording method is a method in which printing is performed by discharging small droplets of ink from fine nozzles and by causing the small droplets of ink to adhere to a recording medium such as paper. A feature of the method is that high-resolution high-quality images can be printed at high speed using a relatively inexpensive apparatus.
A typical ink used in ink jet recording is a dye ink, a pigment ink, or the like. In an ink including a coloring material such as a pigment or a disperse dye, it is important that these coloring materials are stably dispersed on a dispersion medium, but such dispersion is not always easy. In particular, in a case where the temperature conditions with respect to the dispersion system of a coloring material change, the adsorption equilibrium of the coloring material of the dispersant is disturbed, which affects the interactions between the coloring material particles, and thus, with regard to long-term preservation, changes in physical properties occur and/or aggregated contaminations are generated. Such changes in physical properties (in particular, changes in viscosity) in an ink for ink jet printers and/or changes in the characteristics of the head cause clogging of discharge nozzles, and thus, printing is not properly performed in some cases.
To solve such problems, it is necessary to suppress changes in physical properties and/or the generation of aggregated contaminations in the ink by improving dispersion stability or solubility and compatibility of the ink materials, optimizing the ink composition, and the like (for example, refer to Japanese Patent No. 5116002). In addition, attempts have been made to use an ink after removing the collected contaminations by heating the ink in advance (for example, refer to JP-A-3-64376, JP-A-8-73785, JPA-2002-30243, and JP-A-2003-313475).
In general, it is difficult to completely remove the impurities in a production (synthesis) step of a coloring material such as a pigment or a disperse dye, and even in the case of products with a high purity grade, a certain amount of impurities is typically included in a coloring material. As exemplary examples of such impurities, there are coloring matter analogues that are structurally similar to the coloring matter molecules constituting the coloring material. That is, it is difficult to separate structurally similar molecules that during purification of the coloring material. In a case where such a coloring material is dispersed in a dispersion medium, the impurities are eluted into the dispersion medium and are precipitated. Moreover, this phenomenon can occur over time. In the case of a coloring material in which coloring matter molecules have a planar structure, it is thought that the coloring material is dispersed in a dispersion medium in a state in which the coloring matter molecules are stacked with each other, and from such a stacked state, coloring matter analogues are slowly eluted into a dispersion medium. Since the planar structural analogue of the coloring matter molecules exhibits dissolution characteristics and dispersion characteristics different from those of the original coloring matter molecules, in a case where the structural analogue is eluted into a solvent, precipitates different from those of coloring material are generated, these become contaminations, and the contaminations are precipitated or dispersed in the solvent, or float on the solvent.
In addition, the ink in which such a coloring material is dispersed contains a resin for fixing the coloring material onto a recording medium. For example, by being contained in a form of an emulsion, the resin becomes a stable dispersion or solution in the ink. However, in particular, in the ink for industrial applications, there is a case where a large amount of a fixing resin emulsion is included, but with the increase in the content of the fixing resin, there is a growing concern that contaminations derived from the fixing resin will be generated.
Furthermore, in printers for industrial applications, because the operating time is relatively long, the time in which the ink and the members in the printer are in contact with each other inevitably increases. As a result, for example, impurities such as fatty acids (salts) are eluted from the members of the printer into the ink and are precipitated, these become contaminations, and thus, the possibility of causing defects is higher than in the case of the general-purpose printers in the related art.